1. Field of the Invention
The invention relates in general to a signal compensation circuit, and more particularly to a signal compensation circuit of a bus.
2. Description of the Related Art
Nowadays, the PCI (Peripheral Component Interconnect) bus of the notebook is implemented is such layout of daisy chain due to the limited space, wherein the maximum length of the PCI bus is 12 inches. Sometimes, the signals on the PCI bus are twisted or distorted because the layout lengths of the various PCI devices, such as a PCMCIA controller, a LAN (Local Area Network), and a modem card, are different and accordingly the impedance matching are difficult to be reached. Please refer to FIG. 1, it shows the twisting of the signals on PCI buses, wherein the south bridge is connected to the PCI device (1) through PCI device (n) by a PCI bus. The serious distortion of the signal sometimes results in the system malfunction.
Besides the unmatched layout impedance, the transient state of the IC connectors, the variations of the loading, and the un-matching impedance of the PCB (printed circuit board) traces are also the reasons to cause the distortions of the signals. Therefore, it is important to adopt a proper compensating means to maintain the stable operation of the system.
Traditionally, a damping resistor is connected to the bus in series to absorb the reflection energy in order to solve the problem of the signal distortion. On the PCI bus, a damping resistor is inserted between the south bridge and the PCI devices. Then, a testing pattern is inputted to the bus, and the distortion of the signal is controlled within a tolerated range by adjusting the damping resistance. The signal quality on the PCI bus for the mass production is insured after repeatedly testing the damping resistor to select a suitable resistance.
However, the signal on the PCI bus and the loading prior to the mass production can't really reflect the mass production situation well. And the traditional method can't always eliminate the signal distortion effectively. Moreover, the number of damping resistor will be considerable large in the situation that the total lines of the signal controllers, data lines, and the address lines on the PCI bus are over 40, that not only occupy a lot of space on the PCB, but also increase the volume of the PCB, the length of the layout, and the cost of the manufacturing.